I just wanted to say - don't assume that nobody would take the cat.
Years ago my sister was looking for her first cat. She went to a rescue place and was looking at them all. There were some right cuties there. She came across this mean looking black cat who glared at her and hissed.
My sister said "I'll have that one"
The woman looked at her and said "are you sure? she's not very friendly!"
But she wanted her.
They had no kids at that time (siddy didn't like kids!) and she was a hissy, scratchy bugger but also randomly really affectionate. But then she'd bite you in the middle of enjoying a stroke 
Then my sister had a baby and was watching Siddy like a hawk. But she actually was really protective of the baby.
Still bloody insane though
demand a stroke and rip your hand off!
She also used to demand the neighbours let her through their flats so she didn't have to walk round to the grass at the back! She trained the downstairs neighbour to open his door when she yelled and walk after her and open the back window!
One day my sister was outside and picked her up and brought her in, she went loco. hissing and spitting. My sister took her in and came face to face with Siddy! She'd brought in a random black cat. She said "She was hissing and spitting so of course I didn't think anything of it" 
She mellowed a bit in her old age (but not much!
)
Anyway, Siddy died of old age and is now buried in my sister's back garden under a tree, and my sister's husband always has a chat to her when he cuts the grass.
She was a much loved member of the family and had a great life.
Don't think putting your cat to sleep is the only option. The rescue centre didn't think anyone would ever want Siddy.